1. Field of the Invention
This invention relates to railway track circuit energy surge suppressors.
2. Description of the Prior Art
Movement of a railway vehicle along a railroad is necessarily limited to one degree of freedom. That is to say, the vehicle can only travel back and forth along the track. It cannot alter its course to avoid other traffic. In order to prevent railway vehicles on the same track from overtaking each other, a block signalling system has been devised whereby the track is divided into segments, or "blocks," of a length greater than the stopping distance of a train. In order to prevent a problem, only one train is allowed in a particular block at a time. Wayside block indicators positioned before an upcoming block indicate to the locomotive engineer whether or not the block is occupied. If so, the engineer will know to adjust the speed of the train.
The operation of wayside block indicators has been traditionally controlled by the track circuit. The track circuit is essentially an electrical circuit in which the rails in a block complete a connection between an electrical signal transmitter and an electrical signal receiver. Insulating joints may be placed between adjacent blocks to provide electrical separation. When the block is unoccupied, current is allowed to flow through the rails to the receiver. Typically, the receiver acts as a switch to activate the wayside indicators to display an appropriate aspect. If, however, the block is occupied by any part of a train, shunt paths are created by the presence of a wheel and axle set of the train. Thus, current is prevented from reaching the receiver. As such, the wayside indicator will not be activated and will give a stop signal, or simply no signal at all.
In order to make the most efficient use of a track, it is desirable to have a minimum space in excess of stopping distance between adjacent trains. Minimum train spacing is accomplished using an aspect scheme wherein a number of different wayside indicator aspects are displayed. Each aspect represents a condition of operation to which the locomotive engineer should adhere. Examples of such conditions are stop, approach, approach medium, advance approach, and clear.
In order to display a particular aspect, an electrical signal is pulse modulated or otherwise encoded with an appropriate code into the rails by the transmitter. The receiver, which was originally merely a series of relays, interprets the logic of the coded track signal to display the aspect. The track circuit is what is known in the art as a vital circuit. A vital circuit is one in which a malfunction of one of the circuit components cannot give an unsafe condition. Thus, the vital circuit relays were typically very large devices constructed to exacting standards. They had special non-welding contacts to resist sticking when exposed to large current surges, such as by lightning. Because these relays tended to be expensive and slow, however, advances in technology made it desirable to replace them with electronic circuits. The circuits, however, are smaller than the relays, and are more easily damaged by transitory energy surges such as a current surge caused by lightning.
The frequency of lightning surge damage to track circuit equipment depends on a number of factors. First, lightning is a seasonal occurrence. Thus, while lightning is fairly common in the warmer months, it is rarely seen in winter. Additionally, the direction of a lightning storm will effect surge occurrence. For example, a storm passing over a track in a perpendicular direction will have a relatively low probability of causing surge damage. However, a storm moving parallel to the track could strike several consecutive blocks, causing damage in each. This uncertain nature of lightning surge introduces significant logistical and personnel problems into repairing surge damage. Thus, a number of surge suppression devices have appeared in the prior art.
Prior art surge suppression systems have generally comprised an arrester set having three varistor-filled-gap arresters. Two of the arresters are connected line-to-ground and the other is connected line-to-line across the track wires. Sometimes, a secondary stage suppressor has been utilized to supplement the primary arrester set. The secondary stage suppressors, when utilized, are placed for purposes of convenience near the primary arrester set. Occasionally, a tertiary stage suppressor has been placed on circuit boards within the electronic control equipment. These prior art systems have been prone to failure, however.
Some failures of prior art surge suppression systems have been attributed to the design of the suppressor of the secondary suppression stage. In the past, these suppressors have had a tendency to short. This short would increase the current in the secondary suppressor to a level that would cause it to heat and "blow open." Thus, an open circuit would result. All of the surge current not absorbed by the primary arrester would then feed to the equipment. On occasion, this has resulted in significant equipment failures. Moreover, time consuming and expensive on site replacement of the suppressors has been required.